The performance of an internal combustion engine may be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts may be driven by a single crankshaft powered chain drive or belt drive. Engine performance in an engine with dual camshafts may be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft.
U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position on of a camshaft relative to a crankshaft. The control system utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure, PC, on one end of the spool and the relationship between the hydraulic force on such end and an oppositely direct mechanical force on the other end, which results from a compression spring that acts thereon.
U.S. Pat. No. 5,107,804 describes an alternate type of VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing which replace the oppositely acting cylinders disclosed by the aforementioned U.S. Pat. No. 5,002,023. The vane is oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft. The control system of this VCT system is identical to that divulged in U.S. Pat. No. 5,002,023, using the same type of spool valve responding to the same type of forces acting thereon.
U.S. Pat. Nos. 5,172,659 and 5,184,578 both address the problems of the aforementioned types of VCT systems created by the attempt to balance the hydraulic force exerted against one end of the spool and the mechanical force exerted against the other end. The improved control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of the spool. The hydraulic force on one end results from the directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, PS. The hydraulic force on the other end of the spool results from a hydraulic cylinder or other force multiplier which acts thereon in response to system hydraulic fluid at reduced pressure, PC, from a PWM solenoid. Because the force at each of the opposed ends of the spool is hydraulic in origin, based on the same hydraulic fluid, changes in pressure or viscosity of the hydraulic fluid will be self-negating, and will not affect the centered or null position of the spool.
U.S. Pat. No. 5,289,805 provides an improved VCT method which utilizes a hydraulic PWM spool position control and an advanced control method suitable for computer implementation that yields a prescribed set point tracking behavior with a high degree of robustness.
In U.S. Pat. No. 5,361,735, a camshaft has a vane secured to an end for non-oscillating rotation. The camshaft also carries a timing belt driven pulley which can rotate with the camshaft and is oscillatable with respect to the camshaft. The vane has opposed lobes which are received in opposed recesses, respectively, of the pulley. The camshaft tends to change in reaction to torque pulses, which it experiences during its normal operation and it is permitted to advance or retard by selectively blocking or permitting the flow of engine oil from the recesses by controlling the position of a spool within a valve body of a control valve in response to a signal from an engine control unit. The spool is urged in a given direction by rotary linear motion translating means which, is rotated by an electric motor, preferably of the stepper motor type.
U.S. Pat. No. 5,497,738 shows a control system which eliminates the hydraulic force on one end of a spool resulting from directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, PS, utilized by previous embodiments of the VCT system. The force on the other end of the vented spool results from an electromechanical actuator, preferably of the variable force solenoid type, which acts directly upon the vented spool in response to an electronic signal issued from an engine control unit (“ECU”) which monitors various engine parameters. The ECU receives signals from sensors corresponding to camshaft and crankshaft positions and utilizes this information to calculate a relative phase angle. A closed-loop feedback system which corrects for any phase angle error is preferably employed. The use of a variable force solenoid solves the problem of sluggish dynamic response. Such a device can be designed to be as fast as the mechanical response of the spool valve, and certainly much faster than the conventional (fully hydraulic) differential pressure control system. The faster response allows the use of increased closed-loop gain, making the system less sensitive to component tolerances and operating environment.
U.S. Pat. No. 5,657,725 shows a control system which utilizes engine oil pressure for actuation. The system includes a camshaft that has a vane secured to an end thereof for non-oscillating rotation therewith. The camshaft also carries a housing which can rotate with the camshaft but which is oscillatable with the camshaft. The vane has opposed lobes which are received in opposed recesses, respectively, of the housing. The recesses have greater circumferential extent than the lobes to permit the vane and housing to oscillate with respect to one another, and thereby permit the camshaft to change in phase relative to a crankshaft. The camshaft tends to change direction in reaction to engine oil pressure and/or camshaft torque pulses which it experiences during its normal operation, and it is permitted to either advance or retard by selectively blocking or permitting the flow of engine oil through the return lines from the recesses by controlling the position of a spool within a spool valve body in response to a signal indicative of an engine operating condition from an engine control unit. The spool is selectively positioned by controlling hydraulic loads on its opposed end in response to a signal from an engine control unit. The vane can be biased to an extreme position to provide a counteractive force to a unidirectionally acting frictional torque experienced by the camshaft during rotation.
U.S. Pat. No. 6,477,999 shows a camshaft that has a vane secured to an end thereof for non-oscillating rotation therewith. The camshaft also carries a sprocket that can rotate with the camshaft but is oscillatable with respect to the camshaft. The vane has opposed lobes that are received in opposed recesses, respectively, of the sprocket. The recesses have greater circumferential extent than the lobes to permit the vane and sprocket to oscillate with respect to one another. The camshaft phase tends to change in reaction to pulses that it experiences during its normal operation, and it is permitted to change only in a given direction, either to advance or retard, by selectively blocking or permitting the flow of pressurized hydraulic fluid, preferably engine oil, from the recesses by controlling the position of a spool within a valve body of a control valve. The sprocket has a passage extending there through. The passage extends parallel to and is spaced from a longitudinal axis of rotation of the camshaft. A pin is slidable within the passage and is resiliently urged by a spring to a position where a free end of the pin projects beyond the passage. The vane carries a plate with a pocket, which is aligned with the passage in a predetermined sprocket to camshaft orientation. The pocket receives hydraulic fluid, and when the fluid pressure is at its normal operating level, there is sufficient pressure within the pocket to keep the free end of the pin from entering the pocket. At low levels of hydraulic pressure, however, the free end of the pin enters the pocket and latches the camshaft and the sprocket together in a predetermined orientation.
In addition, it is known to have an electronic feedback loop involving sensors sensing the positions of shafts such as camshaft or crankshaft in a VCT system. For example, pulse wheels are rigidly affixed onto the shafts for the sensors sensing purposes. The sensed pulses are in turn processed into information wherein derived positional information of a rotor or vane in relation to a housing is used to control a control valve (spool) which in turn is used to control a phase relationship. Typically, the spool valve comprises two lands thereon for stopping fluid communications as desired.
In Melchior's U.S. Pat. No. 5,645,017, U.S. Pat. No. 5,649,506, and U.S. Pat. No. 5,507,254, a rotary cylinder is connected to and rotates with a drive shaft by means of a gear pinion. A piston having a vane is connected to the driven shaft. One-way communication circuits are provided in the rotary piston, with check valves carried in the vane. The shaft of the piston is hollow and carries a slidable slide that rotates in synchronism with the driving shaft. The slide includes two external recesses that are separated by an axially extending rib that is helical in shape. The unidirectional circuits include a common section with an end leading to an orifice, which depending on the position of the slide is open to the recesses or closed by the axially extending valve rib. When the slide is in the null position, the fluid cannot move between the chambers, in the chambers or out of the chambers. When some leakage has occurred, causing an undesirably or uncontrolled phase shift, the orifice is uncovered or no longer blocked by the axially extending valve rib, allowing a direct one-way fluid flow passage from a first chamber to a second chamber through a check valve to a common passage, through a recess and back to the other passage leading to the second chamber. The shift in fluid from the first chamber to the second chamber causes the piston and axially extending valve rib to rotate relative to the cylinder until the orifice of the common passage is completely obstructed by the axially extending valve rib.
While advancing and retarding of the phase coupling are described as leakage between the chambers, eventually the remaining fluid in the phase coupling will be inadequate to alter the timing between the drive shaft and the driven shaft, due to leakage of the phase coupling as a whole, since a makeup line is not disclosed. The leakage cannot be fixed by moving fluid from one chamber to the other and vice versa, causing the chambers to have an inadequate amount of fluid to properly alter the phase between the drive shaft and the driven shaft.
Melchior cannot provide a makeup source to the chambers. Due to the position of the common passage/orifice and the positioning of the axially extending valve rib, makeup fluid cannot enter the chambers when the phase coupling is in the null position or in other positions based on the unidirectional circuits.
Since the phase coupling in Melchior's U.S. Pat. No. 5,645,017, U.S. Pat. No. 5,649,506, and U.S. Pat. No. 5,507,254 cannot be supplied with makeup oil from a supply due to the axially extending valve rib, the axially extending valve rib cannot be used with phasers that require a constant or semi-constant source of oil pressure to operate, such as a torsion assist phaser, an oil pressure actuated phaser, or a hybrid phaser disclosed infra. In an oil pressure actuated or a torsion assist phaser, the main force in moving the vanes is engine oil pressure, with fluid being supplied to a first chamber and simultaneously exhausted from the other chamber to sump. A constant source of pressurized fluid is required in order to actuate the phaser, and thus alternate the phase. In a hybrid phaser, cam torque is used in conjunction with an oil pressure to actuate the phaser and alter the phase. The oil pressure portion of the phaser is used when the cam torque is not large enough or will not be sufficient to alter the phase. Melchior also discloses a stepped shaped rib with similar problems as described above.